Two positively charged particles $X$ and $Y$ are initially far away from each other and at rest. $X$ begins to move towards $Y$ with some initial velocity. The total momentum and energy of the system are $p$ and $E$.

• A
  If $Y$ is fixed,both $p$ and $E$ are conserved.
• B
  If $Y$ is fixed,$E$ is conserved,but not $p$.
• C
  If both are free to move,$p$ is conserved but not $E$.
• D
  If both are free,$E$ is conserved,but not $p$.